Oxidatively pretreated conductive ceramic for zinc anode

ABSTRACT

The invention relates to an additive to the active mass of a zinc anode for an alkaline secondary electrochemical generator. Said additive contains conductive ceramic powder, preferably titanium nitride particles which is exposed to an oxidation pre-treatment prior to the incorporation thereof into the active mass of the anode. Said ceramic powder is used as electronic conduction in the anode active mass and as zincates retention which are produced by generator discharge. In order to use said retentive capacity, the powder is exposed to an oxidation pre-treatment, whereby making it possible to form the binding sites on the surface of ceramic grains. The inventive additive makes it possible, starting from the first cycles of the electrode formation, to form uniform zinc deposits, thereby increasing the service life for the cycling of the zinc anode.

The present invention relates to the field of electrochemicalgenerators, and more particularly to that of metal-air storage batteriesand systems.

Specifically, it relates to secondary generators with a zinc anode andis intended to obtain a high level of cyclability of the zinc electrode.

The zinc electrode is well-known to the person skilled in the art forits high level of performance. It may furthermore be used in varioussecondary electrochemical systems: alkaline air-zinc, nickel-zinc andsilver-zinc generators, bromine-zinc and chlorine-zinc generators withsaline electrolytes.

Zinc is an attractive anodic active material, having a strongly negativeredox potential of −1.25 V/NHE for the pair Zn/Zn(OH)₂. The zincelectrode offers a theoretical gravimetric specific capacity of 820Ah/kg. It accordingly makes it possible, for example, to achievetheoretical gravimetric specific energies of 334 Wh/kg for thenickel-zinc pair (NiZn), and of 1,320 Wh/kg for zinc-oxygen pair. For anNiZn storage battery, the practical gravimetric specific energy may bebetween approximately 50 and 100 Wh/kg, the voltage furthermore being1.65 volts, instead of the 1.2 volts for other alkaline systems.

Further advantages of zinc which should be emphasised are, on the onehand, its non-toxicity to the environment (production, use, disposal),and, on the other hand, its low cost, which is very much less than thatof other anodic materials for alkaline storage batteries (cadmium andmetal hydrides), or lithium storage batteries.

However, the industrial development of rechargeable systems using a zincelectrode has encountered a major stumbling block, namely theelectrode's inadequate cycle life.

The reactions which occur at the anode are as follows in an alkalinestorage battery:

Recharging of the zinc electrode from the oxides and hydroxides thereofand from the zincates in fact generally gives rise to the formation ofdeposits with a structure which is modified relative to the originalform, said deposits often being described as dendritic, spongy orpowdery. This phenomenon moreover occurs over a very wide range ofcurrent densities.

Successive recharges thus rapidly result in the chaotic growth oroutgrowth of zinc through the separators and in short-circuiting withthe electrodes of the opposite polarity.

As for the powdery or spongy deposits, these prevent the reconstitutionof electrodes capable of satisfactory or extended operation due toinadequate adhesion of the active material.

Furthermore, reduction of the zinc oxides, hydroxides and zincates tozinc at the anode during recharging is also characterised bymorphological changes in the electrode itself. Depending on the mode ofoperation of the storage batteries, various kinds of changes in form ofthe anode are observed as a result of non-uniform redistribution of thezinc during the formation thereof. This may in particular result in atroublesome densification of the anodic active mass at the surface ofthe electrode, most often in the central zone thereof. At the same time,electrode porosity is reduced, which contributes to an acceleration inthe preferential formation of zinc on its surface.

These major shortcomings, which reduce the achievable number of cyclesto a few dozen (a level which is inadequate to ensure economic viabilityfor a secondary system), have given rise to a large number of studiesdevoted to improving zinc deposition characteristics during rechargingwith the aim of increasing the number of charge-discharge cycles whichthe generator can accept.

Various very different approaches have been investigated with theobjective of attempting to minimise or to delay as long as possiblethese zinc formation defects, the following of which may in particularbe mentioned:

-   -   “mechanical” methods intended to reduce the chaotic formation or        outgrowth of zinc, or to avoid powdery deposits: circulation of        the electrolyte and/or of the zinc electrode in dispersed form;        vibration imparted to the electrodes; use of separators which        are resistant to perforation by dendrites, frequently in        multiple layers, and even of ion-exchange membranes, in order to        prevent the migration of zincates;    -   “electric” methods intended to improve the conditions under        which the zinc deposit is formed: control of charging parameters        (intensity, voltage etc.); use of pulsed current, including        current inversion, in an attempt to dissolve the dendrites while        they are forming;    -   “chemical” and “electrochemical” methods: use of additives        incorporated into the electrolyte (fluoride, carbonate, etc.)        and/or into the anodic active material (calcium, barium etc.)        and dilution of the electrolyte, in particular in order to limit        the solubility of the zincates and to form zinc oxide and        insoluble compounds of zinc.

These various methods may be implemented in isolation or in combination.

In any event, they have only limited positive effects which have provedinadequate to impart economic viability to secondary generators with azinc anode and in particular to the nevertheless theoretically veryattractive pair, NiZn; they barely make it possible to achieve or exceedaround a hundred cycles performed with a significant depth of discharge.

Moreover, some of these methods have disadvantageous negative effects,such as:

-   -   increase in the internal resistance of the storage battery (due        to certain additives or to electrolyte dilution),    -   reduction in the life of the nickel cathode (due to the use of        certain additives),    -   mechanical complexity of operation (for circulating systems),    -   increases in the volume and mass of the system (impairment of        specific performance parameters in terms of gravimetric and        volumetric specific energies),    -   increased costs (losing the potential economic advantage).

A major innovation was provided and described by the description ofFrench patent application 99 00859, the developed technology making itpossible to achieve several hundred cycles over a wide range ofoperating conditions and down to very deep depths of discharge thanks tothe implementation of means intended to increase the efficiency of useof the active material by improving the percolation of charges withinit.

The present invention is based on the observation that insufficientdrainage of charges within the active material promotes the formation ofthe zinc deposit during recharging at sites which represent only alimited percentage of the entire active mass. This zinc growth, aphenomenon which most frequently gives rise to a chaotic deposit whichmay result in outgrowths through the separators or in densification ofthe deposit, accordingly proceeds from sites with a limited totalsurface area relative to the overall projected surface area of theanodic material. The technology described in the above-mentioneddocument shows that this mechanism may be greatly reduced if, byincreasing the number of deposit formation sites, the same totalquantity of zinc is deposited over a much larger surface area throughoutthe volume of the electrode.

According to a preferred embodiment, this technology results in the use,within the zinc anode, of two or three levels of electrical collection:

-   -   a main collector network: an electrode support of the “metal        foam” type (reticulate honeycomb structure),    -   a secondary conductor network: a dispersion of conductive,        chemically inert, ceramic particles in the storage battery,    -   a possible complementary tertiary conductor network: a        dispersion of bismuth in the anodic active mass.

An “antipolar mass”, which may consist of nickel hydroxide whenproducing nickel-zinc storage batteries, may also be introduced into thezinc anode and makes a significant contribution to the level ofperformance achieved.

The aim of the present invention is to improve the cyclability of thezinc electrode by prior treatment of the conductive ceramic, before theaddition thereof to the active mass of the zinc electrode, the purposeof the treatment being to impart to said ceramic powder a secondfunction of retaining the zincates formed on discharge of the zincanode.

In the “Journal of the Electrochemical Society”, vol. 145, no. 4, page1211, 1988, C. F. Windisch and al. describe the changes in polisheddiscs of titanium nitride (TiN) which are immersed for 136 days in aconcentrated solution of potassium hydroxide.

Compounds identified as slightly crystallised potassium titanates formon the surface of the material.

As emphasised by J. Lehto in U.S. Pat. No. 6,106,799, it is not alwaysstraightforward to distinguish between titanates and hydrated titaniumoxides, since hydrated titanium oxides may be considered to be amorphousor semi-crystalline forms of titanates.

This same author points out that titanates and hydrated titanium oxideshave ion-exchange properties, which are utilised for treatment ofeffluents containing radioactive ions.

Furthermore, titanium nitrides, in particular in powder form, are notinert with regard to atmospheric oxygen, even at ambient temperature.

Uncrystallised or slightly crystallised titanium oxynitrides andtitanium oxides, which may be detected by XPS analysis, form on thesurface of the grains.

RX analysis of TiN powders does not necessarily reveal the presence oftitanium oxide, but a modification of the nitride lattice parametercorresponding to compounds which are sub-stoichiometric in nitrogen maybe observed.

Commercial TiN powders are generally produced by nitriding titanium.They always exhibit nitrogen deficits of 0.5 to 2% relative to thestoichiometric quantity, which is 22.62% by weight. This deficit may beeven larger when the powders are prepared by methods such as nitridingof titanium dioxide with ammonia, or synthesis by a self-propagatingthermal reaction from titanium oxide, titanium halides, etc.

Titanium oxynitrides are of the general chemical formula TiN_(x)O_(y),with x and y varying between 0.01 and 0.99.

At low oxygen contents, the (face-centred cubic) crystalline structureand the corresponding parameters of titanium oxynitrides are virtuallyidentical to those of the nitride, which makes them difficult toidentify by RX analysis. Titanium oxynitrides are black in colour, andif TiN is golden in colour, progressive oxidation of the TiN results ina colour change starting from bronze to brown and then to black.

This characteristic has been exploited for the preparation of blackpigments as replacements for carbon, iron oxide or manganese dioxidepowders.

Various methods have been described in the literature for preparingtitanium oxynitrides:

-   -   partial reduction of the titanium dioxide by ammonia,    -   partial oxidation of very finely divided TiN,    -   manufacture of TiN by plasma discharge from titanium        tetrachloride.

The oxidation temperature of TiN powders, the process being accompaniedby an increase in weight, is closely related to the nature of thesamples. Fine powders of a diameter of approximately 5 μm begin tooxidise at around 350° C., whereas coarser powders of 50 μm will beginto do so at around 500° C. (P. Lefort and al., Journal of Less CommonMetals, no. 60, page 11, 1978).

Extended, high-temperature treatment of TiN powder results in theformation of rutile-type titanium oxide.

On the basis of this knowledge, the authors of the present inventionthus discovered that oxidation pretreatment of TiN powders, the use ofwhich in a zinc electrode is described in French patent 99 00859,improved the cyclability of the electrode, due to increased reactivityof the ceramic powder with regard to the electrolyte, so giving rise tothe formation of hydrated titanium oxides which may themselves changeinto partially crystalline titanates.

It is thus possible to impart to the ceramic powder, dispersed in theanodic mass, two functions which are essential to the proper functioningof the zinc electrode:

-   -   an electron-conductive function, which will contribute to        obtaining a more uniform distribution of the formation of        metallic zinc within the active mass during successive charging        operations,    -   a retention function for the zincates arising from the oxidation        of the zinc on discharge of the storage battery, thanks to the        formation of adsorbent compounds created by the pretreatment        performed on the TiN powder.

As a result of this pretreatment, which is performed on TiN powderswhich are themselves obtained by various processes, it will be possiblegreatly to accelerate the formation of zincate binding sites broughtabout by surface modification of the TiN in accordance with a mechanismdescribed by Windish and al. In this way, it will be possible, rightfrom the first formation cycles of the zinc electrode, to produceuniform deposits of zinc thanks to said second function imparted to theceramic powders. Producing the most uniform possible deposits of zincwithin the anode right from the first recharge cycles is essential toobtaining a significant cycle life of generators with a zinc anode.

According to the present invention, the oxidising pretreatment must beperformed while avoiding the formation of a significant layer oftitanium oxide, which would bring about an excessive reduction in theelectron conduction of the grains, and would furthermore be totallyinert with regard to the alkaline medium. This pretreatment depends onthe nature and the particle size of the TiN used. The smaller is thesize of the particles, the higher is the reactivity of the TiN withregard to oxygen. For use as an additive for zinc anodes, preferred TiNpowders advantageously have a particle size essentially less than 10microns.

Pretreatment may more readily be performed in air. It is, however, alsopossible to use pure oxygen or mixtures of inert gas and oxygen. Theinert gas may be nitrogen, helium or argon. The oxygen content may bebetween 1% and 99%.

The influence of gas pressure is of little significance, and treatmentmay in particular be performed at ambient pressure in the case of air,or with a slightly elevated pressure in the case of oxygen or syntheticmixtures of gas, said elevated pressure being for example between 0.1kPa and 5 kPa.

The duration of treatment will depend mainly on the composition of thegas (in particular on the oxygen content), the temperature and the grainsize.

Since treatment is performed in the presence of oxygen, for example inair, at ambient pressure, the temperature is between approximately 150and 800° C., for treatment times of between 5 minutes and 15 hours. Itmust be ensured, when treating very fine TiN powders, that temperatureis raised progressively in order to prevent abrupt oxidation of thepowder by ignition.

The titanium nitride powder modified according to the present invention,obtained after such an oxidising pretreatment, may be handled withoutany particular precautions other than those recommended for titaniumnitride powders produced according to the various production processesalready mentioned above.

According to the present invention, the zinc anodes will preferably beproduced according to the production processes described in Frenchpatent applications 99 00859 and 01 10488.

It will be noted in particular that at this stage, according to documentFR 99 00859, the anode has two, or even three, electrical collectionnetworks, which constitute as many sets of potential zinc nucleationsites during recharge, although the conductive ceramic powder is thefirst among these networks:

-   -   the electrode support and charge collector, preferably of the        reticulate metal foam type,    -   the dispersion of conductive ceramic particles in the anodic        mass,    -   a possible dispersion of bismuth within said anodic mass.

Apart from the pretreated TiN particles which here constitute theconductive ceramic and will be the main conductive zincate bindingsites, it may also be useful, as described in document FR 01 10488, tointroduce into the anodic mass additives such as alkali metal oralkaline-earth metal titanates, and also to add to the mixture ofconductive ceramic and titanates, or to the active mass, a quantity ofan additive consisting of at least one compound based on aluminiumand/or calcium, and/or a quantity of an additive consisting of at leastone compound which, on contact with the alkaline electrolyte, formssoluble aluminium compounds, of between approximately 1 and 5% by weightrelative to the zinc oxide.

The electrolyte may also have added to it a quantity of an additiveconsisting of at least one soluble compound of aluminium and/or ofcalcium, of between approximately 1 and 5% by weight relative to thezinc oxide.

The authors have observed that the ion-exchange power of the ceramicspretreated according to the invention is improved by the addition ofaluminium and/or calcium in various forms to the anodic active mass orto the electrolyte.

The zinc anode used may advantageously be of the pasted/plasticisedelectrode type, and thus be produced by pasting, coating or filling byany means, in the liquid or dry phase, of a highly porous,three-dimensional support of the reticulate honeycomb metal foam type,with a paste in particular containing zinc oxide powder, the dispersionof ceramic particles, a plasticiser, and optionally a suspending agent.

By way of non-limiting illustration of the present invention, anadvantageous example of embodiment is described below which makes itpossible to assess the worth of the present invention.

Nickel-zinc storage batteries are produced with pasted/plasticisednickel cathodes within a support of nickel foam. The zinc anodes areproduced by pasting a copper foam support, covered with lead byelectrolytic deposition, of grade 45 PPI (pores per inch) or 18 poresper linear centimetre. The density of the support is 450 g/m².

The active mass for the zinc electrodes is prepared to form a paste ofthe following composition:

-   -   zinc oxide,    -   titanium nitride⁽¹⁾: 15%⁽²⁾,    -   calcium titanate: 2.5%⁽²⁾,    -   bismuth oxide: 5%⁽³⁾,    -   nickel hydroxide: 5%⁽³⁾,    -   plasticiser: PTFE⁽⁴⁾,    -   suspending agent: water.    -   ⁽¹⁾ average particle size: approx. 1 micron    -   ⁽²⁾ by weight relative to the active mass    -   ⁽³⁾ by weight relative to the zinc oxide.    -   ⁽⁴⁾ introduced in the form of a 60%-aqueous suspension, the        concentration of PTFE being 4% by weight relative to the zinc        oxide.

The TiN powder used for the type A electrode is a commercial product.The TiN powder used for the type B electrodes is identical to the powderused for the electrodes A, but has undergone an oxidation pretreatmentof 30 minutes at 250° C. in air according to the present invention.

For all the anodes produced, the solid particles constituting the activemass were subjected to thorough kneading before the addition of water,in order to ensure that they were mixed intimately and as homogeneouslyas possible.

The electrolyte used is potassium hydroxide (KOH) of a concentration of5 N. It is saturated with zincates and contains 10 g/l of lithiumhydroxide (LiOH).

Once introduced within the metallic support, the initial thickness ofwhich is 2 mm, the active mass is dried, and the resultant zinc anode iscompacted under a compaction pressure of 80 kg per square centimetre.Thickness is reduced 0.8 mm.

The electrolyte used is potassium hydroxide (KOH) of a concentration of5 N. It is saturated with zincates and contains 10 g/l of lithiumhydroxide (LiOH).

Open nickel-zinc storage battery assemblies were produced by associatingtwo nickel cathodes with one zinc anode, such that the latter definesthe capacity of the storage battery and the properties thereof may bemonitored during testing.

A combination of two separators is used between the electrodes ofopposite polarity. One is a microporous membrane, such as that offeredfor sale under the brand “Celgard” by the company Hoescht Celanese. Theother is a nonwoven polyamide or polypropylene separator, such as thereference product “FS 2115” from Carl Freudenberg.

The storage batteries produced in this manner are subjected to long-termcycling tests in accordance with standardised methods. The type ofcharge-discharge cycle used, at a set current, is as follows: C/4 mode(charge and discharge each performed in 4 hours, the applied currentcorresponding to one quarter of the nominal capacity of the element)with a depth of discharge of approx. 80%; one cycle comprising totaldischarge (100% depth) is performed every ten cycles.

The type A electrodes retain more than 80% of their nominal capacity for300 to 400 cycles depending on the particular electrode before theircapacity drops off very rapidly.

The type B electrodes retained more than 80% of their nominal capacityfor more than 1,000 cycles, and achieved more than 1,500 cycles with acapacity greater than 70% of nominal capacity. Cycling was stopped after2000 cycles without any abrupt drop in capacity being observed.

It has thus been shown, for the purposes of the present invention, thatthe high levels of performance achieved by adding conductive ceramics tothe anodic mass are very greatly enhanced by subjecting the TiNparticles to an oxidising pretreatment, said pretreatment beingperformed at elevated temperature in the presence of oxygen and leadingto the formation of titanium oxides and oxynitrides on the surface ofthe ceramic particles.

On contact with the alkaline electrolyte of the zinc anode generators,said titanium oxides and oxynitrides may in turn change into partiallycrystallised titanates so forming zincate binding zones and imparting tothe ceramic particles their second function of retaining zincates as thesite of formation thereof (by oxidation of metallic zinc on discharge).

This function is in addition to the primary electron conduction functionof the ceramic particles, which latter function promotes the in situreduction of said zincates into metallic zinc when the generator ischarged.

This twin function of the pretreated ceramic additive according to theinvention thus makes it possible to avoid over the very long term themorphological changes to the zinc anode which are usually observed andwould result in a short cycle life.

Without departing from the scope of the present invention, the inventionmay be performed by being associated with some or all of the additivesor charging procedures described in the literature and applied to theuse of zinc electrodes.

Naturally, and as will be clearly evident from the above, the inventionis not limited to the specific embodiments which have been described byway of example. The invention is not limited to the stated examplesthereof, but encompasses all variants.

1. An additive to the active mass of a zinc anode for an alkalinesecondary electrochemical generator, comprising a conductive ceramicpowder which has undergone an oxidising pre-treatment before beingincorporated into the active mass of said electrode.
 2. A conductiveceramic powder used such as an additive to the active mass of a zincanode for an alkaline secondary electrochemical generator according toclaim 1, wherein it comprises titanium nitride particles.
 3. Aconductive ceramic powder used as an additive to the active mass of azinc anode for an alkaline secondary electrochemical generator accordingto claim 1, wherein the titanium nitride particles have a particle sizeessentially less than 10 microns.
 4. A conductive ceramic powder used asan additive to the active mass of a zinc anode for an alkaline secondaryelectrochemical generator according to claim 1, wherein the oxidisingtreatment was performed in the presence of oxygen.
 5. A conductiveceramic powder used as an additive to the active mass of a zinc anodefor an alkaline secondary electrochemical generator according to claim1, wherein the oxidising treatment was performed in air.
 6. A conductiveceramic powder used as an additive to the active mass of a zinc anodefor an alkaline secondary electrochemical generator according to claim1, wherein the oxidising treatment was performed at a temperature ofbetween 150° C. and 800° C.
 7. A conductive ceramic powder used as anadditive to the active mass of a zinc anode for an alkaline secondaryelectrochemical generator according to claim 1, wherein the oxidisingtreatment was performed for a period of between five minutes and fifteenhours.
 8. A conductive ceramic powder used as an additive to the activemass of a zinc anode for an alkaline secondary electrochemical generatoraccording to claim 1, wherein the titanium nitride was obtained bynitriding titanium.
 9. A conductive ceramic powder used as an additiveto the active mass oi a zinc anode for an alkaline secondaryelectrochemical generator according to claim 1, wherein the titaniumnitride was obtained by nitriding titanium dioxide.
 10. A conductiveceramic powder used as an additive to the active mass of a zinc anodefor an alkaline secondary electrochemical generator according to claim1, wherein the titanium nitride was obtained by a self-propagatingthermal reaction from titanium oxide or titanium halides.